Since 1983, when Holm published his technique for transperineal interstitial implantation of radioactive seeds into the prostate (J Urol 1983; 130:283-6), prostate brachytherapy has grown into an industry. Selected prostate cancer patients are now routinely counseled regarding brachytherapy as a treatment option. By virtue of the fact that more than 30% of newly diagnosed cancers in men arise in the prostate, prostate brachytherapy has become an important procedure. Certain technical aspects of the procedure, such as radiation dosimetry and ultrasound technology, have improved and/or are better understood than in 1983. However, the implant needles upon which physicians rely to deliver radiation to the prostate have not kept pace. This is because devices and techniques are unable to overcome pubic arch interference, the most common problem facing the prostate brachytherapist.
As shown in FIG. 1, a substantial portion of the prostate gland 10, typically the anterolateral portion of the prostate 10a, shown by the x's, (from the clinician's perspective, looking towards the supine patient's head from below), may sit behind the pubic arch 11. This pubic arch 11 is formed by the convergence of the right 12 and left 13 pubic bones at the midline. This pubic arch 11 is closer to the perineum 14, than the prostate 10. The rectum 15 is located posteriorly.
Standard prostate brachytherapy is performed with the patient supine in the lithotomy position. As shown in FIG. 2A, the patient's legs (not shown) are suspended in stirrups. A needle 16 is employed, and a rectangular template (template grid) or needle guide 17 is placed against the perineum 14. The needle guide 17 rests on a support, which holds an ultrasound probe (not shown) that is inserted into the rectum 15. This ultrasound probe permits visualization of the prostate 10 during the procedure. The ultrasound support, in turn, rests on a stand or brace that is locked in place during the actual implant so that the ultrasound probe can be moved forward and back in relation to a defined position in space.
The needle guide 17 has a parallel array of holes extending therethrough, for accommodating the needle 16. These holes are perpendicular to the template's vertical surface. Once the prostate volume and location have been confirmed on step section ultrasound planimetry, implant needles 16 are guided through the appropriate holes in the template 17 to the desired location within or around the prostate 10 in order to fulfill the brachytherapy plan.
However, as shown in FIG. 2A, the needle 16, as inserted through the needle guide 17, may not reach the target prostate area 10a, as it encounters the pubic arch 12. This is known as pubic arch interference, and may arise from patient positioning, patient anatomy or operator equipment orientation.
Pubic arch interference is frequently an insurmountable obstacle for even the most experienced brachytherapist. Needle displacement or blockage by bone can lead to significant loss of radiation dose coverage of the prostate. In one published series, Peschel from Yale University reported that 25% of his patients had pubic arch interference which disrupted the implant plan and drastically lowered disease-free survival rates at four years post implant (J Brachyther Intl 1998; 14: 197-8). Similarly, Wallner from Memorial Sloan-Kettering reported that approximately 20% of his patients were at risk for prostate gland underdosage because of bone interference (Wallner, J Urol 1991; 146:90-5).
Nearly all patients with localized prostate cancer could be candidates for integration of prostate brachytherapy into their treatment protocol. It is typically administered as the sole form of radiation therapy, or can be given in conjunction with external beam radiotherapy. However, some patients are precluded from undergoing prostate implantation for technical reasons. Chief among the contraindications is pubic arch interference.
Pubic arch interference is highly variable between patients, and is only loosely related to the size of the gland. Patients with a very small pelvic inlet may be difficult to implant despite a small gland volume. Conversely, patients with a large pelvic inlet may be easy to implant despite a large gland volume. The overriding issue is whether the pubic arch extends beyond the lateral and anterior margins of the prostate gland. If so, then it becomes extremely challenging, and sometimes impossible, to insert needles into the shielded regions of the prostate.
As stated above, pubic arch interference can be assessed via Computerized Tomography (CT) or ultrasound scan prior to the implant procedure in order to determine whether the pelvic bones might impede needle insertion. The largest prostate cross-section is overlaid on the narrowest section of pubic arch, and the overlap is measured. A rule of thumb is that if more than 25%, or one centimeter, of the prostate cross-section is blocked, the odds of achieving a successful implant are questionable (Bellon, IJROBP 1999; 43:579-81). The American Brachytherapy Society conducted a survey among brachytherapists and learned that prostate size greater than 60 grams was felt to be a relative contraindication to prostate brachytherapy alone (Nag, J Brachyther Int 1997; 13:243-51). It subsequently published consensus guidelines for clinicians recommending that implanting glands larger than 60 grams should not be attempted by novice brachytherapists because of the technical difficulties caused by pubic arch interference (Nag, IJROBP 1999; 44:789-99).
The seasoned brachytherapist can employ several maneuvers upon encountering pubic arch interference in order to skirt the pelvic bones and circumvent the pubic arch. This type of troubleshooting would permit implanting of the shielded portions of the prostate, thereby preserving the intended radiation dose distribution. The most basic maneuver, as shown in FIG. 2B involves withdrawing the needle and reinserting it into a neighboring hole in the needle guide 17. Here, the needle 16 reaches the prostate 10. Brachytherapy of the contacted portion of the prostate 10 is now possible, but the anterolateral portion of the prostate 10a, is left untreated or insufficiently treated.
Alternately, in this situation, the clinician can also slightly redirect the needle 16 using the bevel on the needle tip to cause the needle to diverge towards the desired location. If necessary, the needle 16 may be diverted after it has passed through the needle guide 17, but before it has entered the patient in a further effort to achieve the desired targeting. The needle tip 16a can be bent to deflect the needle towards the target location, but this can make it difficult to push the seed sources through, once the desired location is reached.
Repositioning the patient in an extended lithotomy posture, whereby the legs are drawn closer to the patient's head, can expand the space between the prostate and the pubic arch enough to allow accurate needle placement. The orientation of the template, in relation to the patient's perineum, can be modified by tilting the ultrasound support in an effort to bypass the obstructing bone. However, both of these methods may present severe discomfort or the potential for injury to the patient.
Finally, freehand needle placement can be attempted using various angles by removing the template grid. Again, this procedure still runs the risk of pubic arch interference.
Most brachytherapists have adopted the technique of modified peripheral seed source loading in order to minimize central high dose areas in the prostate. This has been done to protect the urethra. This style of seed implantation relies heavily on accurate seed placement in the outer portions of the prostate gland to generate the prescribed radiation dose. Therefore, avoidance of pubic arch interference is critical if one is to achieve a successful implant.
While pubic arch interference presents one of the greatest difficulties in brachytherapy, other factors also contribute to degradation of the intended dose during the implant procedure. These include patient motion, instability of the ultrasound stand or brace, poor ultrasound image quality, needle divergence, seed settling or migration, and misplacement of seeds in the bladder or rectum.
U.S. Pat. No. 2,269,963 (Wappler), U.S. Pat. No. 4,700,692 (Baumgartner), U.S. Pat. No. 5,242,373 (Scott), U.S. Pat. No. 5,860,909 (Mick), U.S. Pat. No. 5,928,130 (Schmidt), and U.S. Pat. No. 6,007,474 (Rydell) reflect devices that are employed to implant radioactive seeds into tissue. None addresses the problem of pubic arch interference. None of the implant needles currently available, including those described in U.S. Pat. Nos. 5,938,583 (Grimm) and 6,210,315 (Andrews), or those marketed by Mentor, Mick (MTP-1720-C, MTP-1820-C), Med-Tec (MT-BRACHYTHERAPY-5001-25, MT-BRACHYTHERAPY-5051-25), Best (Flexi-needle), Bard (BrachyStar®), or MD Tech offer a solution to the problem. The prostate stabilization needles in U.S. Pat. No. 4,799,495 (Hawkins) used during prostate brachytherapy to immobilize the gland do not help the brachytherapist avoid the pubic arch. Neither the real time brachytherapy spatial software registration and visualization system outlined in U.S. Pat. No. 6,129,670 (Burdette), nor the prostate brachytherapy software planning engine recently described in U.S. Pat. No. 6,200,255 (Yu), provides a solution to pubic arch interference despite a sophisticated approach to seed implantation. Finally, in U.S. Pat. No. 6,027,446 (Pathak), there has been devised a method for assessment of pubic arch interference, but has not offered a remedy.
There is a substance in current use in medicine which possesses properties that, when adapted to brachytherapy, may be exploited to overcome pubic arch interference. Nickel-titanium alloys, commonly known as Nitinol®, show a very pronounced shape memory and superelastic effect. Shape memory characteristics allow it to stay in a deformed shape until heated, whereupon it returns to its pre-deformed shape. For example, a surgical hook may be deformed into a straight configuration at room temperature and recover its hooked shape upon introduction to tissue, which is above room temperature. The superelastic characteristics of Nitinol® allow a hook to be constrained within a straight cannula during insertion into tissue, only to immediately regain its curved shape upon deployment into the tissue. Recovery of its original shape during unloading is the unique aspect of nickel-titanium alloys that is responsible for its integration into many medical inventions, eg. U.S. Pat. No. 5,000,912 (Bendel), U.S. Pat. No. 5,011,473 (Gatturna), U.S. Pat. No. 5,219,358 (Bendel), and U.S. Pat. No. 6,033,404 (Melzer).